Generally, an image sensor is a semiconductor device that converts an optical image to an electrical signal and they can be classified as either a charge coupled device (CCD) image sensor or a Complementary Metal Oxide Silicon (CMOS) image sensor (CIS). The CMOS image sensors adopt a switching system by providing a photodiode and MOS transistor within a unit pixel and then implements an image by detecting electrical signals of unit pixels sequentially. In a related CMOS image sensor, a photodiode area for converting an optical signal to an electric signal and a transistor area for processing the electric signal are arranged in a horizontal plane.
In a horizontal type CMOS image sensor, a photodiode and a transistor are formed adjacent each other on a substrate in a horizontal direction. However, in this type of arrangement, an additional area for forming the photodiode is utilized. Therefore, a fill factor area is reduced and the possible resolution is limited.